_TableView : Improve sizing behaviour
_TableView : Improve sizing behaviour

- Default `minimumVisibleRows` constructor argument to 0.
- Fix bug in disconnecting from `prevModel.dataChanged`.
- Respond to addition and removal of columns.
- Don't add space for a horizontal scrollbar if it is always off anyway.

andrewkaufman/gaffer,boberfly/gaffer,appleseedhq/gaffer,andrewkaufman/gaffer,appleseedhq/gaffer,lucienfostier/gaffer,GafferHQ/gaffer,GafferHQ/gaffer,hradec/gaffer,ImageEngine/gaffer,lucienfostier/gaffer,boberfly/gaffer,andrewkaufman/gaffer,johnhaddon/gaffer,hradec/gaffer,hradec/gaffer,lucienfostier/gaffer,ImageEngine/gaffer,johnhaddon/gaffer,appleseedhq/gaffer,johnhaddon/gaffer,lucienfostier/gaffer,ImageEngine/gaffer,hradec/gaffer,johnhaddon/gaffer,ImageEngine/gaffer,GafferHQ/gaffer,hradec/gaffer,johnhaddon/gaffer,boberfly/gaffer,GafferHQ/gaffer,boberfly/gaffer,andrewkaufman/gaffer,andrewkaufman/gaffer,GafferHQ/gaffer,appleseedhq/gaffer